A simple voice coil actuator is an ideal solution for many applications requiring precise movement, such as semiconductor equipment, defense systems and life-sustaining medical systems due to the simple, non-contacting structure of the design. The structure is typically the same as that found in a simple speaker.
The voice coil actuator is a direct drive, limited motion device that utilizes a permanent magnetic field and a coil winding (conductor) to produce a force proportional to the current applied to the coil. The permanent magnetic field is provided by a permanent magnetic housing containing one or more permanent magnets, while the coil winding is a part of a coil assembly that moves in-and-out of the permanent magnetic housing along the axis thereof.
The Lorentz principle governs the electromechanical conversion mechanism of a voice coil actuator. This law of physics states that if a current-carrying conductor is placed in a magnetic field, a force will act upon it. The magnetic flux density, “B”, the current, “I”, and the orientation of the field and current vectors determine the magnitude of this force. Further, if a total of “N” conductors (in series) of length “L” are placed in the magnetic field, the force acting upon the conductors is given by: F=KBLIN, where K is a constant. Hence, the force applied between the coil assembly and the permanent magnetic housing is proportional to the amount of current flowing through the coil.
For voice coil actuator applications, it is desirable to measure the motion, position and/or acceleration of the coil assembly with respect to the permanent magnetic housing when a current of certain magnitude is applied. Due to the strong magnetic field in the voice coil actuator, linear variable displacement transducers (LVDTs) are not suitable for such measurements.
Currently, potentiometers and optical sensors are used with the voice coil actuator, but they have their own shortcomings. By way of example, using potentiometers, variable resistors or other contact sensors will turn the voice coil actuator into a contact device, which is limited by the lifecycle due to wear and tear of the contacts. In addition, much noise is generated under vibration due to the use of contact fingers. Further, optical sensors must be mounted externally to the voice coil actuator, and is very costly.
Therefore, it is desirable to provide a non-contact sensor that can be embedded within the voice coil actuator to measure a movement between the coil assembly and the permanent magnetic housing, which is substantially impervious to the strong magnetic field in the voice coil actuator.